Keyboards provide a well-known device for entering data into data processors or other systems requiring a means for entering data. Typical keyboards include a plurality of key switches and have a two-dimensional scanning matrix. Each cross over point in the matrix has a possibility of a corresponding key switch which may be a saturable core-type solid state analog switch. The scanning matrix includes drive lines which scan along one axis of the matrix and sense lines which scan along the other axis.
In a known prior art system as shown in U.S. Pat. No. 4,390,866, the drive lines provide a signal which is sequentially applied to the input of the key switches while a multiplexer scans the sense lines. A common counter is used to keep the multiplexer synchronized with the circuitry providing the sense line signals. The counter also provides a code assignment data for each of the key switches. The code assignment data is advanced as each of the key switches are scanned.
In this known system, when a key is actuated, a signal on the drive line is inductively coupled to the corresponding sense line. The multiplexer, in turn, applies this signal to a differential amplifier. The output of the differential amplifier is applied to a threshold detector, the output of which is coupled to other circuitry including a shift register for storing the differential amplifier output signal. The shift register provides a strobe signal to the data processor. When the strobe signal is generated, this manner, the data processor receives the code assignment data corresponding to an actuated key.
When a saturable core or another type of analog key switch, such as a capacitively coupled switch, is employed, its output signal level which is a function of the key switch position. As the key switch is depressed, the output signal level approaches the threshold level. Any mechancial or electrical noise signal present may vary the output signal above and below the threshold for a number of scanning cycles, and thus cause multiple strobing. The multiple strobing will be interpreted by the data processor as though a particular key was repeatedly actuated several times, even though the key was actuated only once.
The multiple strobing results from the fact that the point of travel of the key switch at which actuation is recognized is the same point of travel at which deactuation is recognized. This makes the output signal of the key switch particularly susceptible to noise as it approaches the actuation recognition point. This problem is further complicated by the fact that differential amplifiers and shift registers are especially susceptible to electromagnetic interference or noise.
Such a system also exhibits a lack of adaptability, in that a designer must choose the threshold value, and once chosen, this value cannot be changed. Thus, factors such as temperature and humidity cannot be taken into account. Furthermore, there is no method of compensating for key-to-key noise variations because the threshold level is the same for all keys.